Embodiments of the invention relate generally to couplings between a structure and a flowline, such as a fluid transfer line extending between two floating structures, a riser suspended from a floating structure, or a riser extending from the seafloor. More particularly, embodiments of the invention relate to apparatus and methods for coupling the flowline to the structure such that the flowline is rotatable relative to the structure.
Flowlines are typically used to transfer fluid between two floating structures, or between a floating structure and a fixed structure. For example, the gravity actuated pipe (GAP) developed by Single Buoy Moorings, Inc. is a deepwater fluid transfer system including one or more neutrally buoyant flowlines suspended between two structures, at least one of which is floating. Each flowline is coupled at each end to a support frame, which is, in turn, coupled to one of the structures by a connecting chain. Flexible jumpers, each connected between a flowline and one of the structures, transfer fluid to or from the structures, depending on the direction of fluid flow through the flowline.
The flexible nature of the jumpers and connecting chains permits limited movement of the flowlines relative to the structures between which they are coupled in response to surrounding water currents and motions of the floating structure(s). Although this relative motion is desirable, repeated movement of these components causes them to be susceptible to fatigue damage that may limit the service life of the GAP system. In particular, the connecting chains and flowlines, proximate their midpoint, experience fatigue due to cyclic bending and flexing.
Risers are often used to transfer fluid between the seafloor and a floating structure. The riser may be coupled to the floating structure by a joint assembly. The joint assembly includes a stiff cylindrical can that radially surrounds a portion of the riser and is coupled by means of supports or guides to the hull of the floating structure. To alleviate high stresses to the riser which would result from directly coupling the riser to the cylindrical can, the joint assembly further includes a stress joint coupled between the upper end of the riser and the cylindrical can. In operation, the joint enables limited angular movement of the riser relative to the floating structure caused by surrounding water currents and/or motion of the floating structure. Loads, and associated stresses, imparted to the riser are transferred from the riser through the stress joint to the supports, bypassing the upper end of the riser and thus preventing high stresses in the riser at this location. Although the joint assembly may be an effective coupling means, manufacturing the assembly, in particular the stress joint, is costly.